In various embodiments, the description may refer to power supply devices adapted to be used, for example, together with light sources, such as LED light sources.
FIG. 1 in the annexed drawings shows, by way of a reference example, the design of a power supply device of the switching type.
The device in FIG. 1, denoted on the whole by 10, allows to supply, from a mains alternating voltage AC IN, for example 220V/50 Hz or 110V/60 Hz, a load L comprised for example of one or several LEDs (for example a so-called LED “string” adapted be used as a source of lighting radiation).
Device 10 is built around a transformer T, comprising a primary winding T1 and a secondary winding T2.
Primary winding T1 identifies the “primary” side of device 10, which substantially comprises a rectifier 12, a stabilizing network 14 and at least an electronic switch 16 (e.g. a mosfet) adapted to be alternately switched on and off on the basis of a control signal supplied by a controller 18.
On the secondary side of device 10 (which is therefore isolated from the primary side), beside the “main” secondary winding T2, adapted to supply load L, an auxiliary secondary winding denoted by 20 may be provided, which is adapted to supply circuitry components, such as logic circuits 22 and 24 which ensure the supply to the logic that controls the operation of device 10 (primarily through controller 18).
The exemplary embodiment shown in FIG. 1 has circuits 22 and 24 coupled to each other through an optocoupler (opto-isolator) 26a, 26b, so as to ensure, for the control signal as well, the galvanic isolation of both parts of device 10, that relate respectively to primary winding T1 and to secondary winding T2 of the transformer.
The diagram in FIG. 1 has a merely exemplary function, and it merely aims at introducing in general a possible circuitry design which may be subjected to the phenomena better detailed in the following.
The scope of the present invention, and in particular the various embodiments, are therefore in no way limited to the specific circuit arrangement of FIG. 1, which basically is shown again in the diagram of FIG. 4, as will be explained in the following.
In a device such as shown in FIG. 1, it may be crucial to ensure that the supply function to circuits 22, 24 has a stable voltage level.
This is done for example, when load L varies, by preventing changes (for example in voltage) in circuits 22 and 24 that may cause an improper operation of circuit 10.
This is true regardless of the specific circuit topologies shown in FIG. 1. In this respect it will be appreciated that several of the elements, denoted in the figure by the usual symbols indicating electrical components, have not been described in detail: this choice aims at highlighting that, for the purpose of implementing the embodiments, the particular circuit solutions shown may be replaced by functionally equivalent solutions, which are well-known to an expert in the field.
This is substantially true for FIG. 2 and FIG. 3 as well, which show block diagrams that, on the basis of the inventors' observations, could be used to ensure a stable supply for circuits 22 and 24 even if load L changes.
For example, the diagram in FIG. 2 shows the possibility to add a linear regulator 20a to auxiliary winding 20 already shown in FIG. 1.
This solution has the drawback of an extremely low efficiency: linear regulator 20a must be able to face voltage steps across secondary winding 20a and the corresponding multiplication factors of auxiliary current.
Another intrinsic drawback of the arrangement in FIG. 2 is that, because of the fixed coupling of both secondary windings T2 and 20, when the voltage across secondary winding T2 drops, the voltage across auxiliary secondary winding 20 can reach very low levels, below the minimum value needed to supply auxiliary logic 22, 24.
Such a problem may arise in the presence of loads comprised of LED light sources L. In particular, we refer to LED “strings”, i.e. loads with a constant power consumption in spite of a possible change of the output voltage, due to a change of the number of active LEDs within the string, e.g. because one or several such LEDs are shunted, i.e. virtually short-circuited, in order to vary the brightness of the light source.
In the presence of a lower number of active LEDs, auxiliary circuit 20, 20a is no longer able to yield a sufficient voltage to supply secondary logic 22, 24.
The inventors have observed that, as is schematically shown in FIG. 3, another option would be to arrange a buck converter at the main secondary winding T2, while feeding the supply voltage for logic 22, 24 across a charged capacitor 23, according to the typical arrangement of a buck converter, through an inductor 25 connected to secondary winding T2 via an electronic switch such as a metal oxide semiconductor field effect transistor (MOSFET) 27, with reference 29 denoting the diode for recirculating the current through inductor 25 (and capacitor 23) when switch 27 is closed.
When mosfet 27 is on (i.e., conductive), such an arrangement of a buck converter feeds the load and charges inductor 25, while in the “off” stage, i.e., when mosfet 27 is non-conducting or open, current recirculates in the load and in diode 29.
The solution considered in FIG. 3 has the advantage of a good efficiency. This advantage, however, is diminished by the presence of a rather high number of components (with a consequent increase in costs) and by the need, in the case of a high voltage output, of a rather costly electronic switch, such as a mosfet 27.
The solution considered in FIG. 3 can also show operation problems with low values of output voltage. Actually, in such conditions, the buck circuit which lowers the voltage cannot work, because the supply voltage, i.e. the output voltage, is lower than the voltage needed for the auxiliary supply.